1. Field of the Invention
The present invention relates to the separation of various difficulty separable close-boiling impurities such as esters, aldehydes, ketones and hydrocarbons from propylene oxide by treatment with solid activated carbon adsorbent.
2. Description of the Prior Art
Alkylene oxides such as propylene oxide are highly important chemicals useful in a great number of applications. An important commercial technology for producing the alkylene oxides is via the catalytic reaction between the corresponding olefin and an organic hydroperoxide, the hydroperoxide being prepared by hydrocarbon oxidation. See, for example, U.S. Pat. No. 3,351,635.
Ethylene oxide is generally prepared by silver catalyzed molecular oxygen oxidation of ethylene. The lower alkylene oxides contain impurities which are difficult to separate due to small differences in boiling point between the alkylene oxide and the impurities. In the case of propylene oxide, for example, considerable effort has been devoted to separating close boiling methyl formate, acetaldehyde, propionaldehyde, C.sub.4 -C.sub.7 hydrocarbons especially C.sub.6 hydrocarbons, and other impurities. Ethylene oxide has associated with it impurities such as formaldehyde while butylene oxide generally is contaminated with impurities such as methyl acetate, ethyl acetate, acetone, methyl ethyl ketone, methanol and hydrocarbons, especially C.sub.6 -C.sub.8 hydrocarbons.
One direction taken by prior workers has been to provide extractive distillation techniques to accomplish the separation. U.S. Pat. No. 3,838,020 shows a dual solvent extractive distillation process. U.S. Pat. No. 3,843,488 shows extraction distillation using a C.sub.8 to C.sub.20 hydrocarbon to purify propylene oxide. U.S. Pat. No. 3,909,366 shows extractive distillation purification of propylene oxide using C.sub.6 to C.sub.12 aromatic hydrocarbon. U.S. Pat. No. 4,140,588 uses water in extractive distillation purification of propylene oxide. U.S. Pat. No. 3,881,996 uses plural stage distillation to purify propylene oxide. East German Patent Specification 118,873 uses aliphatic alcohols such as tertiary butanol in separating methyl formate from propylene oxide by extractive distillation. U.S. Pat. No. 5,006,206 uses tertiary butyl alcohol and water in the extractive distillation purification of propylene oxide.
It has previously been proposed to separate oxygen-containing impurities from the propylene oxide by extractive distillation using lower glycols such as ethylene glycol and propylene glycol. See U.S. Pat. No. 3,578,568 which describes this procedure and which teaches use of solvent in amount to comprise 15 to 50% of the vapor space in the distillation zone. U.S. Pat. No. 5,000,825 describes a similar separation but one which uses much lower solvent concentrations whereby propylene oxide losses are reduced.
U.S. Pat. No. No. 3,477,919 teaches a method for purifying propylene oxide contaminated with impurities such as methyl formate which boil near propylene oxide. The methyl formate impurity is removed from the contaminated propylene oxide by reaction with an aqueous slurry of calcium hydroxide.
U.S. Pat. No. 2,622,060 teaches a process for separating propylene oxide from a crude reaction mixture by treatment with an aqueous alkali metal hydroxide solution.
U.S. Pat. No. 2,550,847 teaches a process for the purification of propylene oxide in a crude reaction mixture containing methyl formate by subjecting the mixture to strong agitation with an aqueous solution of an alkaline saponifying agent.
U.S. Pat. No. 3,350,417 teaches a process for purifying propylene oxide comprising parallel and serial stages of distillation and a caustic treatment to simultaneously aldolize acetaldehyde and saponify methyl formate. The solvent used in the reaction step is removed before subsequent caustic treatment.
U.S. Pat. No. 4,691,034 removes methyl formate from propylene oxide by contact with an aqueous calcium hydroxide slurry to which a solubilizer has been added. U.S. Pat. No. 4,691,035 removes methyl formate from propylene oxide by contact with a base such as sodium hydroxide in water and glycerol.
A problem with prior procedures which involve distillation separation of impurities such as C.sub.6 hydrocarbons from propylene oxide is that, at the elevated temperatures necessary to carry out the distillation, additional impurities are formed. For example, at the distillation temperatures, non-volatile high molecular weight ethers are formed which require a separate treatment for removal.
U.S. Pat. No. 4,692,535 shows such removal of non-volatile high molecular weight ethers from propylene oxide by treatment with an absorbent such as activated carbon, for example.
U.S. Pat. No. 3,904,656 shows treatment of an ethylene oxide stripper bottoms with ion exchange materials to remove metal salts and with activated carbon to remove ultra-violet light absorbers. The stripper bottoms treated are primarily comprised of water and ethylene glycol and usually contain at most extremely minor amounts of ethylene oxide, e.g. less than 0.1 wt. %.
Despite the efforts of prior workers, work has continued in an effort to further improve the separation of close-boiling contaminating impurities such as C.sub.4 -C.sub.8 hydrocarbons, esters such as methyl formate and methyl acetate, aldehydes such as acetaldehyde and propionaldehyde, ketones such as acetone and methyl ethyl ketone, alcohols such as methanol, and the like from propylene oxide.